The invention relates to the operation of an internal combustion engine in combination with an NOx storage catalytic converter and an NOx sensor which is mounted in the flow direction behind the catalytic converter. The three-way catalytic converter no longer satisfies the requirements as to exhaust-gas quality in the area of combustion with a lean air/fuel mixture (xcex greater than 1). Here, NOx storage catalytic converters are used for gasoline engines as well as for diesel engines. These catalytic converters store nitrous oxide which is emitted in lean engine operation. Stored nitrates are released and reduced to nitrogen by the operation of the engine in the rich region (xcex less than 1).
Ideally, the engine is driven lean in a first phase until the NOx storage catalytic converter is full; that is, until the catalytic converter can no longer store any further nitrous oxide. Thereafter, and in an ideal manner, a second phase follows having a rich operation for that time span which is needed for the regeneration of the NOx storage catalytic converter. The storage capability of the NOx catalytic converter in the lean operating region is dependent upon charge and drops continuously. If the first phase lasts too long, then unwanted nitrous oxide emissions occur. A second phase which takes too long has the consequence of increased HC and CO emissions. Accordingly, the problem is present that the change between the two phases is so undertaken that neither increased NOx emissions nor HC emissions occur.
In this connection, it is known from U.S. Pat. No. 5,473,887 to estimate the NOx quantity, which is stored in a first phase, by a summed engine rpm and to then change into the second phase when the summed engine rpm exceeds a predetermined threshold value. The stored quantity is thereby modeled. Its actual value is not detected. The modeling can simulate the actually stored quantity only incompletely. For this reason, the first phase is either too long or too short with the above-mentioned disadvantageous consequences for the quality of the exhaust gas.
U.S. patent application Ser. No. 08/254,603, filed on Aug. 12, 1997, (PCT/DE 97/01714) discloses an NOx sensor for monitoring an NOx storage catalytic converter and for measuring its actual level of charge which makes possible a quasi-linear control for the storage of NOx. When the measurement of the actual storage charge level indicates an exhaustion of the storage capacity, a rich pulse is generated, that is, an operation of the engine with a rich mixture for regenerating the storage catalytic converter. SAE technical paper 960334 discloses an NOx sensor having an approximately linear signal characteristic.
It is an object of the invention to provide a control of the regeneration of a storage catalytic converter with the aid of an NOx sensor which comes closer to the ideal of an optimal usage of the NOx storage capability by a complete filling and discharging of the storage catalytic converter while simultaneously reducing the unwanted HC and CO emissions.
Here, the start, the duration and the height of the rich pulse are to be optimized. A rich pulse which is too long increases the HC and CO emissions. A rich pulse which is too short effects an incomplete emptying of the store which leads away from the objective of a storage phase as long as possible with fuel-economic lean engine operation.
The method of the invention is for changing the mixture composition of the exhaust gas supplied from a combustion process to an NOx storage. The method includes the steps of: providing an NOx sensor mounted rearward of the NOx storage viewed in flow direction; changing the exhaust gas composition in a first phase so as to cause the exhaust gas to contain more NOx than in a second phase; changing the exhaust gas composition in the second phase so as to cause the exhaust gas to contain a reducing agent; and, changing over from the first phase to the second phase when one of the following conditions is satisfied: the signal of the NOx sensor reaches a first threshold value (UT) which indicates a filled NOx storage; or, a signal formed by considering the signal of the NOx sensor reaches a first threshold value (UT) which indicates a filled NOx storage.
The control of the air/fuel ratio during operation of an internal combustion engine having a catalytic converter and an exhaust-gas sensor takes place in such a manner that a lean operation and a rich operation of the engine are ultimately controlled. The exhaust-gas sensor is sensitive for nitrous oxide and is mounted rearward of the catalytic converter in flow direction. The engine is driven with a lean mixture in time average.
The change between both phases is controlled by the NOx sensor signal in a first embodiment. The NOx sensor detects the NOx concentration in the exhaust gas. If this concentration exceeds a predetermined threshold, the storage regeneration is triggered by the mixture enrichment.
In a second embodiment, a change takes place on the basis of the NOx mass flow rearward of the NOx storage catalytic converter. The NOx mass flow can be determined from the NOx sensor signal and the exhaust-gas mass. The exhaust-gas mass can be computed from the intake air quantity ml and the fuel mass.
In both embodiments, a disturbed sensor signal can lead to an unwanted exceeding of the threshold value. An unwanted exceeding of the threshold value occurs when the sensor signal indicates a higher NOx concentration than is actually present. To avoid an unwanted change, the triggering of the mixture enrichment takes place when the time integral of the NOx concentration or of the NOx mass flow exceeds a predetermined threshold value. The integral formation affords the advantage of an independence from the operating point of the engine because, when forming the integral, the air mass ml, which is inducted by the engine, goes into the formation of the integral. The air mass ml participates in the determination of the exhaust-gas mass flow. A further advantage compared to a triggering of the regeneration by the instantaneous values of the NOx concentration or NOx mass flow lies in a greater insensitivity with respect to sensor disturbance influences.
In the context of a further embodiment, an enrichment of the mixture takes place when the level of conversion of the NOx storage catalytic converter drops below a predetermined threshold. This method affords the advantage that it is especially referred to the NOx storage catalytic converter because of the formation of the conversion.
The duration of the regeneration is advantageously so computed as a function of the supplied quantity of the reducing agent and the temperature that the regeneration is sufficient for completely reducing the stored nitrous oxide to nitrogen. The computed duration of regeneration is advantageously checked via measurement on the basis of an NOx sensor signal in the subsequent storage phase. In this embodiment, the NOx sensor serves to correct the computation parameters and thereby to adapt the computer module to the actual conditions which change, for example, because of deterioration.